1. Field of the Invention
The invention relates to a defect detection apparatus for an exhaust gas sensor. More particularly, the invention relates to a defect detection apparatus and a defect detection method that detect cracks in an element of the exhaust gas sensor.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-A-8-327586 describes an apparatus that detects the defect of an exhaust gas sensor provided in an exhaust passage for an internal combustion engine. More specifically, the apparatus detects the defect of an air-fuel ratio sensor that generates an output corresponding to the oxygen concentration in exhaust gas.
In the aforementioned apparatus, a reverse voltage (i.e., a negative voltage) is applied to the air-fuel ratio sensor at predetermined time intervals so that an electric current flows in the air-fuel ratio sensor in a direction opposite to the normal direction of flow (hereinafter, the electric current flowing in the air-fuel ratio sensor is referred to as “sensor current”). Based on the value of the sensor current when the reverse voltage is applied, it is determined whether a defect, such as a crack in the element, is present in the air-fuel ratio sensor. According to this method, the defect of the air-fuel ratio sensor can be detected easily and accurately.
In the apparatus, the value of the signal output from the air-fuel ratio sensor is not normal during a time period after application of the reverse voltage starts. Therefore, it is not appropriate to use the signal output from the air-fuel ratio sensor in an engine control (for example, the feedback control of the air-fuel ratio) during the time period. That is, if a diagnostic process is performed when the engine control is executed, the engine control may be adversely affected. However, no measures are taken to prevent the engine control from being adversely affected. Thus, the apparatus needs to be improved.